This invention relates to an improved process of making heat-resistant, friction products such as brake shoe linings and the like.
For a long time, asbestos fibers have been the dominant fiber used in making brake shoe linings and like products. The success of asbestos was largely due to its excellent heat resistance and other physical properties which it imparted to the finished products. The material had substantial additional processing advantages as became readily apparent to those investigators who attempted to find suitable substitutes for asbestos. For example, asbestos fibers can be viewed as bundles which tend to open up to form additional fibrous surface and help bind an ostensibly dry molding composition together. One important way in which this characteristic was utilized included a preforming step wherein asbestos-reinforced compositions were compressed to a shaped article having sufficient density and green strength to facilitate its subsequent handling and processing into a finished product. A number of fiber substitutes have been suggested for asbestos. These include such materials such as the slag material disclosed in U.S. Pat. No. 4,150,011 to Searfoss and Jones, the basalt rock product disclosed in U.S. Pat. No. 3,896,075; metal fibers; and the like. In general, these fibrous substituents resisted the traditional preforming techniques, or yielded pre-molds which were of an inconveniently low green strength, or required undesirable modification of the subsequent molding step.
A number of techniques were tried in an attempt to provide a suitable process for utilizing the slag fibers. These involved the necessity of using relatively slow processing procedures and/or the use of chemical additives. In either case, the resulting process was unwieldy and expensive.